The present invention relates to a method and apparatus for horizontal continuous casting with a stationary guide mold and one or a plurality of pairs of gripping rolls actuated or driven by a DC-motor for the stepwise or incremental extraction or withdrawal of the continuous casting or cast strand from the mold.
In horizontal continuous casting of metals, in particular of alloys having an elevated melting point, the casting or strand, which has not yet completely solidified, is withdrawn from a stationary or an oscillating mold. In such a case the withdrawal or extraction of the casting occurs discontinuously, i.e. stepwise or incrementally. Depending on the process, either there is a pause between the individual extraction steps or the casting is pushed back over a small distance. The pushing back of the casting is intended to take into account, on the one hand, the thermal expansion of the casting, and, on the other, to effect a solid welding or fusion of the individual casting skin or shell sections formed within the continuous casting mold.
Two different basic arrangements are known for the withdrawal of the casting or strand from the mold. The first arrangement consists of jaws that engage or grip the casting and move it over a small distance. When the grip of the jaws on the casting is released and the jaws are returned to their original position, they reengage the casting, whereupon the motion cycle is repeated. This type of drive is inaccurate in view of the fact that it is necessary to move large masses, i.e. the jaws, thus requiring correspondingly large gear drives or transmissions that have a rather substantial lost motion or relatively great amount of play. Moreover, there exists the problem that the jaws will not always grip the casting at exactly the same spot, thereby producing additional inaccuracies during the extraction of the casting.
The exact movement pattern or motion cycle during the withdrawal of the casting or strand is of particular significance, not only because of production rate considerations, but also for the achievement of a high-quality continuous-casting product. If the withdrawal of the casting occurs in such a way that there does not occur any welding or fusion between the freshly formed casting skin or shell ring and the balance of the casting skin or shell, and if the casting with its casting skin is extracted further, the skin will rupture and the molten mass will emerge or break-out from the mold, whereupon the continuous casting process has to be interrupted immediately.
A more accurate control or regulation of the extraction or withdrawal of the casting can be brought about if gripping rolls directly engage the casting. The increased accuracy is due to the fact that during such a withdrawal of the casting or strand, there exists a constant contact between the casting and the gripping rolls, which contact constitutes a prerequisite for the accurate extraction or withdrawal of the casting or strand.
From German Pat. No. 1,583,611 there is known a device for the intermittent removal of a metallic casting from a mold, wherein a pair of feed rolls driven by a transmission alternatingly engage the casting and additionally perform a reciprocating motion along the casting axis. To achieve reproducibility of the casting motion, there is additionally provided a clamping device for the casting. This device is of relatively complex design and does not permit a completely accurate motion cycle, so the drawbacks already referred to still arise.
In German Pat. No. 2,340,636 there is disclosed a device for the stepwise extraction of a casting from a horizontal continuous-casting mold in which a pair of gripper rolls is actuated by a gearing or transmission driven by a stepwise operating DC-motor. For control of the motor its speed is determined in an analog manner and the angle of rotation of the gripper rolls is determined in a digital manner. The motion of the gripper rolls cannot be inferred directly from the speed of rotation of the motor in view of the fact that the system for the transmission of forces between the motor and the gripper rolls has some free motion or play. Through digital determination of the movement of the gripper rolls, however, it is possible to determine the motion of the casting which, in turn, serves for adjustment of the motor speed. In addition, this device is designed so that when a predetermined torque is exceeded a pause in the casting withdrawal motion is initiated in order to wait for the detachment of the ring of the casting skin from the mold wall. This device therefore has a complex design and is thus markedly susceptible to trouble or malfunction. As a result, in many instances the required operational safety is not provided.
German Pat. No. 1,783,032 describes a device for continuous horizontal casting by means of a stationary guiding mold and a DC-motor-actuated or driven pair of gripper rolls for performing the stepwise or incremental extraction of the casting from the mold. The DC-motor is connected with a control device which, through programmed periods and sequences, transmits pulse sequences to the DC-motor in programmed durations and sequences. The power transmission proper takes place, however, not through the DC-motor, but through an electro-hydraulic stepping motor. The term "electro-hydraulic stepping motor" is to be understood to include the combination of an electric stepping motor and a hydraulic motor, with the electric stepping motor actuating a control valve that controls the supply of pressure medium to the hydraulic motor. By controlling the pressure with the valve there can be brought about a control of the speed of the hydraulic motor. While this arrangement has the necessary torque for the withdrawal of the casting, the required accuracy of extraction or withdrawal of the casting does not always exist because fluids are not fully incompressible, thus causing inaccurate step sequences. Also this equipment is very costly.